Abstract
In two-dimensional (2D) materials family, pure element thin layer such as graphene, silicene, and typical compound film exampled by transition-metal dichalcogenides (TMDs) all exhibit attractive physical characteristics covering from metals, semiconductors to insulators. In particular, single-layer (SL) heterojunctions, termed as the interface between dissimilar materials, realize one-dimensional (1D) electronic systems at their hetero-interfaces, which expectedly give rise to new and interesting properties and promise new applications. Molecular-beam epitaxy (MBE), which is known for its precise control in deposition coverage (thickness), can be a great technique in pursuit of new material and heterostructures. In this study, ultrathin β-phase Te films are firstly fabricated on highly oriented pyrolytic graphite (HOPG) by MBE. While for thicker films, the cell size is found more consistent with that of the [1010] surface of bulk Te crystal. Scanning tunneling spectroscopy (STS) measurements suggest the β-tellurium films are semiconductors with energy bandgaps narrowing with increasing film thickness and predominantly occurring at the valence-band maximum (VBM). The latter can be explained by the strong coupling of states at the VBM but a weak coupling at conduction band minimum (CBM) as revealed by density functional theory (DFT) calculations. Then, a diverse interface structure is achieved by depositing WSe2 and MoSe2 sequentially by MBE. It reports a strong anisotropic behavior of the hetero-interface formation process. Specifically, a sharp interface is obtained only when WSe2 deposition precedes MoSe2 (denoted as WSe2-MoSe2), whereas an alloy (Mo1-xWxSe2) without a noticeable boundary between the two materials is found when MoSe2 is grown first. The process is not very temperature sensitive and can be attributed to an 'edge segregation' effect and supported by the first-principles total energy calculations. Besides, the electronic bands and their alignment at the hetero-interfaces are examined by STS, revealing the type-II alignment for both monolayer (ML−ML) and ML−bilayer (ML-BL) lateral junctions irrespective of the presence or not of step states.
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